Ground Control Points 101

What are ground control points? Why do they matter? How are they used?

With the growing constellation of commercial earth-observation satellites, there has been an associated growth in the inclusion of spatial data in our everyday lives. Once the realm of surveyors and rocket scientists, Global Positioning Satellite (GNSS) technology is now in our automobiles, wrist watches, and cell phones, and is becoming increasingly tied to image data, be it a picture of the Earth or the front of the restaurant we just made cell phone reservations with while stuck in traffic.

To enable such generalized use of aerial, satellite and ground-based imagery, geospatial professionals process the collected images with ground coordinates in order to correctly position the imagery in relation to the Earth.

To ensure that the image is in the right position and that the correct image of the restaurant comes up on the screen of the onboard navigation system in our car, it is necessary to provide accurate ground coordinates to tighten the accuracy of the overhead imagery.

This is the role of CompassData. We are continually collecting new Ground Control Points (GCPs) across North America and around the world to provide accurate coordinate data to support an increasing variety of applications.

Reliable GCPs are an essential input for precise orthorectification of:

Remotely sensed imagery

LiDAR and IFSAR Surface Models

Image and mapping data quality assessment

Photogrammetric mappingand

A fundamental building block of Geographic Information System (GIS) development

Bringing the Real World to Remotely Sensed Imagery

In order to be of any use spatially, Airborne and Spaceborne imagery must be referenced to the real world. By recording precise coordinates of photoidentifiable locations within an image, those coordinates form the basis for remapping that image, through the use of robust software such as Esri ArcGIS, into a usable, accurate map image, ideal for a variety of applications and industries.

Without ground control, accuracies for spaceborne images are typically 10-50 meters off, based on onboard position and attitude sensors and known orbital parameters.

Once ground control has been applied, those accuracies improve to .5-2 meters or better.

Through the use of ground control and sophisticated terrain modeling techniques, aerial images can be generated into orthophoto maps with sub-foot relative accuracies and absolute accuracies of under 2 feet.

So, contrary to popular belief, higher quality aerial or satellite imagery cannot replace the need for ground control. In fact, GCP collection becomes increasingly more important as image accuracy is required and user expectations increase. Fortunately, collecting ground control is now a much faster, more accurate, and cost-effective process thanks to the use of GNSS technology.

Field Data Collection and Remotely Sensed Ground Control

There are two ways to ensure that Ground Control Points are accurate. One is to have a human being physically at the location of the required control point, who uses GNSS-enabled equipment with mapping or survey grade capabilities, to verify the longitude, latitude, elevation, and position relative to photoidentifiable elements around it (buildings or other unmovable objects).

CompassData has highly trained field data collectors all over the world, continually adding to and updating our archive of off-the-shelf GCPs.

The other way is through Remotely Sensed Ground Control Points (RSGCP) which are now obtainable through a highly sensitive and accurate satellite radar imagery coupled with postprocessing done using our archive of confirmed GCPs across the globe.

For GCPs in very remote or dangerous parts of the world, RSGCPs are now an ideal option.

As GIS/GNSS technology grows, so will GCP need an increase

Just a few years ago, not many people had heard of photogrammetry, GIS imagery, remote sensing, or LiDAR, except geospatial professionals. These terms may still be somewhat rarified, but awareness is growing daily.

As more and more industries and individuals learn about how mapping can improve their business and their lives, the demand for better accuracy will grow. Along with accuracy, the requirement for standardized deliverables that can be quickly and easily ingested into GIS databases will rise.

CompassData is at the forefront of this industry and has been since the early 1990s. All of our more than 50,000 Ground Control Points include a complete set of standardized, accurate deliverables that make our GCPs not only the largest commercially available data set of control points in the world but also the easiest to use.

Before CompassData, there were no standards for commercial Ground Control Points. We were one of the first to create the business model, so we set the standards – almost by default.